Signals on optical fiber transmission lines characterized by large bit rate distance products, such as undersea or transcontinental terrestrial lightwave transmission systems and which employ optical amplifiers are subject to a host of impairments that accumulate along its length. The source of these impairments within a single data channel include amplified spontaneous emission (ASE) noise generated in the Erbium-Doped Fiber-Amplifiers (EDFAs), polarization dependent gain caused by hole burning in the EDFAs, polarization dependent loss (PDL) in the passive components, nonlinear effects resulting from the dependence of the refractive index of single-mode fiber on the intensity of the light propagating therethrough, and chromatic dispersion which causes different optical frequencies to travel at different group velocities. In addition, for wavelength division multiplexed (WDM) systems in which a plurality of optical channels are transmitted on the same optical fiber, crosstalk between channels caused by the fiber's nonlinear index or incomplete channel selection at the receiving terminal must be considered. Generally speaking, the impairments that limit the system's performance cause two types of degradation in the received eye pattern; random fluctuations in the bit energy (caused by noise) and non-random pulse shape distortions. Distortions of the second type are sometimes referred to as Inter-Symbol Interference or ISI. As the bit rates rise into the gigabit per second range it becomes critical to manage those impairments that effect the shape of the received pulses, and to limit the ISI.
Methods of reducing noise and distortion in single channel lightwave transmission systems include the use of the NRZ format with synchronous polarization, phase, and amplitude modulation (U.S. Ser. No. 60/012,453, filed Feb. 28, 1996) dispersion management of the transmission line. and/or the use of optical solitons. As discussed in U.S. Pat. No. 5,526,162, modulating the state-of-polarization of the optical carrier at the bit-rate of the transmitted NRZ signal (a so-called synchronous modulation technique) can greatly improve the transmission performance of long-haul optical amplified transmission systems. In addition to the synchronous polarization modulation, superimposed amplitude and phase modulation can dramatically increase the eye opening of the received data pattern. As discussed in Bergano, et al., "100Gb/s Error Free Transmission over 9,100 km using Twenty 5 Gb/s WDM Channels," OFC '96, paper PD23, San Jose Calif., February 1996, these synchronous modulations techniques were used in a WDM demonstration having a total transmission capacity of 100 Gb/s (20 WDM channels at 5 Gb/s) over 9,100 km. Another approach to reducing noise and distortion employs soliton pulses, which essentially result from the selection of a particular amplitude modulation and dispersion map.
In patent application, Bergano 20 filed Dec. 20, 1996, a method and apparatus is provided that yields improved performance of optical transmission systems by synchronous modulation of the transmitted signal's amplitude. An amplitude modulator receives an optical signal onto which data has been modulated at a predetermined frequency. The modulator re-modulates the amplitude of the optical signal in a continuous fashion with a waveform that is periodic and whose fundamental frequency is equal to the same predetermined frequency at which the data is modulated onto the optical signal. The resulting signal (which is neither a pure NRZ or RZ signal) is more tolerant to the distortions usually found in lightwave transmission systems, thus giving superior transmission performance. For a single channel transmission line, the waveforms of the various modulations imparted to the optical signal may be optimized for the particular characteristics of the transmission line.
However, when the previously mentioned technique is applied to WDM systems transmitting a plurality of optical channels on the same fiber, optimum performance may not be achieved because each optical channel experiences different transmission line characteristics (due to the dispersion slope or a lack of flatness in the EDFA gain). As a result of these differing characteristics, re-modulating the data-modulated signal with a uniform waveform across all channels is problematic.